We have studied factors controlling cyclic nucleotide phosphodiesterase in rat renal cortex. The cyclic GMP phosphodiesterase activity, but not the cyclic AMP phosphodiesterase activity, is decreased in rats treated with dexamethasone. In addition an endogenous activator of phosphodiesterase, located in particulate fraction, is released by osmotic shock and can activate a cAMP phosphodiesterase and a cGMP phosphodiesterase. The activator differs from a previously described protein activator of phosphodiesterase in that it is heat labile and does not require calcium for activity. It can also activate phosphodiesterase from other sources, including rat liver. On sucrose density gradient centrifugation the activated form of activator-sensitive enzymes has a lower sedimentation velocity than does the unactivated form, suggesting that the activated form has a lower molecular weight. This is consistent with the interpretation that the activator is a protease.